This invention relates generally to a method and apparatus for producing high quality continuous tone and/or color images on photosensitive material (i.e., photographic paper or film) from information provided in digital form. More particularly, this invention relates to a method and apparatus for controlling a plurality of light sources to enable them to produce high quality continuous tone images on photosensitive material.
In the field of photographic digital printers and image setters, the use of multiple light sources to simultaneously expose multiple individual pixels is well known. U.S. Pat. No. 3,988,742 describes using light emitting diodes (LED""s) and fiber optic tubes to deliver light to the photosensitive material. Applications of this technology have included typesetting and the generation of lithographic films for printing. In these applications, each LED is typically coupled to the input end of a fiber optic tube and the output ends of multiple tubes are arranged in a linear array. As photosensitive material is transported past the linear array, a digital controller energizes the LED""s to form a latent image on the photosensitive material. This process is described in U.S. Pat. Nos. 3,832,488 and 4,000,495 and 5,093,682.
In typical typesetting and lithographic applications, it is generally unnecessary to provide for continuous tonal values. Rather, each light source is generally driven to saturate the photosensitive material. Exposure in excess of what is needed for saturation has essentially no effect. Accordingly, in such printing systems, it is only necessary to drive the light sources to such an extent that the dimmest light source saturates the photosensitive material. Thus, there is generally no need to precisely match the outputs of the multiple light sources.
Continuous tone images are predominately comprised of intermediate tonal values of color or gray tones. The formation of high quality continuous tone images by multiple light sources requires that the sources be precisely matched. Failure to match the light sources, at all useable light levels, will typically produce unwanted lines or artifacts in the printed image. U.S. Pat. No. 3,988,742 generally recognizes the need for normalization of light sources to minimize mismatches.
Photosensitive materials exhibit two well known and undesirable properties called (1) latent image degradation and (2) reciprocity failure. Latent image degradation causes the image exposed onto photosensitive material to change between the time the material is exposed and the time it is processed. Reciprocity failure causes photosensitive materials to respond differently to identical amounts of illumination depending on the duration and intensity of the illumination. For example, a first exposure of X intensity for Y seconds may produce a different result than a second exposure of X/2 intensity for 2Y seconds. These two properties can produce unwanted artifacts to appear in the printed image.
The present invention is directed to a method and apparatus for normalizing the output of multiple light sources used to expose a photosensitive material in order to print high quality continuous tone and/or color images.
In accordance with one aspect of the invention, each of the multiple light sources is driven by a different signal weighted to cause all of the light sources to output light of intensity appropriate to produce a uniform image density over a wide range of image densities.
In accordance with a preferred embodiment of the invention, each light source is driven by a different digital-to-analog converter (DAC). A control processor supplies a multibit intensity correction value to each DAC enabling each DAC to supply a drive signal to its associated light source weighted to cause all of the light sources to output light of substantially uniform intensity at a certain intensity level.
In accordance with the preferred embodiment, each light source is further driven by a different timer circuit. Exposing efficiency data for each light source is stored in an exposure normalization lookup table which is accessed by the processor to supply a multi-bit exposure time correction value to each timer circuit to control the on-duration of its associated light source. This allows each light source to produce a normalized exposing efficacy over a wide range of print density levels.
In accordance with another aspect of the invention, the apparatus is used to produce a test image pattern which is then scanned to determine the relative exposing efficacy of each light source. This exposing efficacy data is used to build the aforementioned normalization look up tables.
In the preferred embodiment, the exposing efficacy of each of the multiple light sources is measured by first exposing a predetermined test image pattern onto photosensitive material. The photosensitive material is then processed to reveal the image. The image is then scanned into a computer for subsequent analysis. The test image is configured to enable the computer to determine which of the multiple light sources exposed each particular area of the test image. Upon analysis of the test image, the computer assesses the relative exposing efficacy of each of the multiple light sources. This is done by measuring the image density in multiple areas corresponding to each of the light sources. Based on the density variations within the image, the intensity correction values to be applied to the DACs, and the exposure time values to be applied to the timer circuits are calculated. The test image is then reprinted and the analysis process repeated until a visually satisfactory hard copy test image is produced.
The properties of latent image degradation and reciprocity failure can produce visible artifacts in the image proximate to the edges of a scan strip. That is, light sources close to the edge of a scan strip typically become more effective at higher densities than the light sources in the middle of the scan. By individually normalizing the light sources in accordance with the invention over the range of image densities within the printing spectrum, this variation can be effectively eliminated.